1. Field of the Invention
The present invention relates to a hologram recording medium having a hologram recording material layer suitable for volume hologram record. The present invention relates in particular to a hologram recording medium having a hologram recording material layer suitable for record/reproduction using a blue laser light.
2. Disclosure of the Related Art
Research and development of holographic memories have been advanced as large-capacity recording technique making high-speed transmission possible. O plus E, vol. 25, No. 4, 385-390 (2003) describes basic structures of holographic memories and a coming prospect thereof.
Examples of the property required for a hologram recording material include high refractive index change at the time of recording, high sensitivity, low scattering, environment resistance, durability, low dimensional change, and high multiplicity. About holographic memory record using a green laser, various reports have been made hitherto as follows.
As a hologram recording material, there is known a photopolymer material made mainly of an organic binder polymer and a photopolymerizable monomer. However, the photopolymer material has problems about environment resistance, durability and others. In order to solve the problems of the photopolymer material, attention has been paid to an organic-inorganic hybrid material made mainly of an inorganic matrix and a photopolymerizable monomer, and the hybrid material has been investigated. The inorganic matrix is excellent in environment resistance and durability.
For example, Japanese Patent No. 2953200 discloses a film for optical recording wherein a photopolymerizable monomer or oligomer and a photopolymerization initiator are contained in an inorganic substance network film. It is also disclosed that the brittleness of the inorganic network film is improved by modifying the inorganic network organically. However, the compatibility between the inorganic substance network and the photopolymerizable monomer or oligomer is bad. Therefore, a uniform film is not easily obtained. A specific disclosure of the publication is that a photosensitive layer having a thickness of about 10 μm (par. [0058]) is exposed to an argon laser having a wavelength of 514.5 nm (par. [0059]).
JP-A-11-344917 discloses an optical recording medium wherein an organic-inorganic hybrid matrix contains an optically active monomer. In the organic-inorganic hybrid matrix, a metal element has an alkyl group (a methyl group) or an aryl group (a phenyl group). However, the introduction of the methyl group makes it impossible to improve the compatibility between the hybrid matrix and the optically active monomer. The introduction of the phenyl group gives a more improvement in the compatibility than the introduction of the methyl group. However, the introduction of the phenyl group causes a fall in the curing speed of a hybrid matrix precursor ([0015] in the above publication). A specific disclosure of the publication is that record is made in a hologram recording layer having a thickness of 100 μm, using a YAG laser having a wavelength of 532 nm (Example, [0031]).
JP-A-2002-236439 discloses a hologram recording material comprising: a matrix made of an organic-inorganic hybrid polymer obtained by copolymerizing an organometallic compound containing an ethylenically unsaturated double bond and an organic monomer having an ethylenically unsaturated double bond, as main chain constituting components, and/or a hydrolyzed polycondensate thereof; a photopolymerizable compound; and a photopolymerization initiator. By the introduction of the large organic main chain component into the matrix material, the compatibility between the matrix and the photopolymerizable compound is improved. However, the introduction of the large organic main chain component permits the presence of a two-component structure of the organic main chain and an inorganic network in the matrix material. Thus, it appears that the matrix may not exhibit unified behavior at the time of recording so as to cause nonuniform recording. If the ratio of the organic main chain component in the matrix is large, the same problems as in the case of the above-mentioned photopolymer material, which uses an organic binder polymer, are caused. A specific disclosure of the publication is that a hologram recording material layer having a thickness of 20 μm (par. [0080]) is exposed to an argon laser having a wavelength of 514.5 nm (par. [0081]).
In order to solve the problems of the hologram recording materials disclosed in the above-mentioned individual publications, JP-A-2005-321674 discloses a hologram recording material comprising: an organometallic compound at least containing at least two kinds of metals (Si and Ti), oxygen, and an aromatic group, and having an organometallic unit wherein two aromatic groups are directly bonded to one metal (Si); and a photopolymerizable compound. In Example 1 of the publication (in particular, pars. [0074] to [0078]), it is disclosed that a hologram recording medium which has a layer of the above-mentioned hologram recording material having a thickness of 100 μm gave a high transmittance, a high refractive index change, a low scattering, and a high multiplicity in record using a Nd:YAG laser (532 nm).